The suggestion that arising neoplastic cells are eliminated early by T-cell-mediated immune mechanisms (immune surveillance) has been supported by both direct and indirect clinical and laboratory observations. Recently, however, this proposition has been challenged on theoretical and experimental grounds. The congenitally athymic ('nude') mouse offers a unique opportunity to test the immune surveillance hypothesis. Using this animal as a target for chemical tumorgenesis by urethan, a well-defined, short-acting carcinogen, the incidence of tumors will be determined following pre-, peri-, and postnatal induction by low doses of the drug. A possible immunosuppresive effect of urethan will be studied by quantitating cellular and humoral immune responses in drug-treated, thymus-bearing animals. In addition to influencing tumor incidence, immune surveillance also may manifest itself by selectively eliminating tumor cells of relatively high antigenicity. It follows that tumors arising in nude mice may be more immunogenic than those developing in thymus-bearing animals. Such antigenic differences will be examined by use of an in vitro cytotoxicity assay for cell-mediated immunity in mice injected with isolated tumor cells.